In many such prior-art systems, the wave-engaging elements are hulls floating in the water, in which cases the said oscillatory rotations may be called "relative pitch" or "relative roll" between one hull and the next. It is a characteristic of hinged-hull systems, and of some other types as well, that they have a well-defined position of equilibrium in which they rest whenever there are no waves. The said oscillatory rotations occur about this rest position, first in one sense and then in the other. If a pair of hinged hulls be considered straight when at rest, they will become concave downward as the crest of a wave passes, then straight at the wave's quarter-length point, then concave upward as the trough passes, then straight again at the other quarter-length point. Thus each wave flexes a hinge both up and down. The resulting angles of relative pitch vary from wave to have and are not large in most weather, 20 degrees from rest position being greater than average even in the absence of any resistance to the hinging action.
All the said systems provide some means to convert the said variable wave-induced oscillations into mechanical shaft power, thence usually to electric power. The conversion is direct in some systems, while others use an intermediate hydraulic or other type of stage. A wide variety of said means is found in the prior art, said means being a patentable feature of some prior-art inventions.
While the variety of said means is wide in the prior art, there is by contrast much less variety in the manner in which the means is installed. The prior art can in fact be said to have evolved a standard method or arrangement for installing such means, one used in a majority of hinged-hull systems. This method is illustrated by, among others, the following:
Hillson, U.S. Pat. No. 882,883, FIGS. 1 and 2. PA1 Casella, et al, U.S. Pat. No. 917,411, FIGS. 1, 2 and 5. PA1 Tornkvist, U.S. Pat. No. 4,036,563, FIG. 9. PA1 Cockerell, British Pat. No. 1,448,204, the drawing. PA1 1. As a single wave passes, the energy-absorbing element goes through a single cycle of length change. When said element's line of action is above the hinge axis, for instance, the passage of a wave crest lengthens the element, while the passage of a trough shortens it, both changes being relative to said element's length with the system at rest. PA1 2. The amount of length change by said element is, approximately, directly proportional to the angle of relative pitch between the two hinged hulls. In particular, if said element were a hydraulic cylinder and piston so valved as to function as a double-acting force pump, the arrangement pictured by Cockerell, it would deliver an amount of liquid nearly proportional to the hulls' angle of relative pitch.
which, with their respective verbal disclosures, are hereby incorporated by reference into this application.
In these references, each said conversion means includes an element whose two ends are pivotally connected, one end to one and the other end to the other, of a hinge-connected pair of hulls. As the hulls pitch, the distance between the points of connection fluctuates, thus changing the length of said element of said conversion means. Said means resists changes to the length of said element, so that work is done by the hulls on said element. Said means is arranged either to dissipate work done on said element, or to transmit it elsewhere.
Said element thus has a definite line of action, defined by its points of connection to the two hulls. In all the foregoing references, and in others as well, the line of action of said element lies well clear of the axis of the hinge which connects the two hulls. As the references show clearly, this is true when the hulls are at rest, and it remains true when the interhull joint is deflected by waves.
This separation between the line of action of the work-absorbing element and the axis of the interhull hinge is a remarkably stable feature of the prior art. In the four references cited above, this feature appears in association with three different kinds of work-absorbing elements. In Casella, et al, there are at least two such lines of action, both well clear of the hinge axis, one above and the other below. In Tornkvist's FIG. 9, the hinge-connected wave-engaging members are not hulls in the ordinary sense of the word, but the standard arrangement for work-absorbing elements appears nonetheless.
The said standard arrangement of installations of energy-absorbing elements gives the associated systems two characteristics which are noteworthy, because the present invention changes them:
It is also worth mentioning that the said standard arrangement lends itself to the use of robust, serviceable, proven hardware. In particular, said arrangement is compatible with use of a hydraulic reciprocating force pump as the work-absorbing means. The loads imposed on the piston and cylinder of such a pump by the said standard arrangement are all axial, thanks to the pivotal end connections. This is desirable, because sidewise loads between piston and cylinder, or between piston rod and seal, promote wear and shorten service life.
A departure from the standard arrangement just described was recently made by Hagen in U.S. Pat. No. 4,077,213, which is hereby incorporated by reference into this application. Hagen's objective is to maximize the efficiency of a system which converts power from water wave to hydraulic to mechanical to electric forms, in that order.
To produce alternating current for general use, it is necessary that the mechanical rotor of its generator be run at constant speed, and such a constant speed is desirable for other types of electric power generation as well. This imposes a constant speed of rotation on the hydraulic-to-mechanical conversion machine. A simple, reliable machine of the latter type is the Pelton wheel, which also is suitable for high pressure drops. The Pelton wheel is highly efficient as long as its speed and inlet pressure are compatible. With the speed restricted to being substantially constant, it becomes desirable to hold the inlet pressure constant also.
This means that the wave-to-hydraulic power converter should deliver substantially constant pressure. Where said converter is a hinged-hull system using the standard prior-art arrangement of its energy-extraction element, and where the system is so regulated that said converter delivers substantially constant pressure, then the energy obtained from a wave will be directly proportional, other things equal, to the wave's height. This is because the motions of the hulls, and of most other types of wave-engaging member, are almost directly proportional, other things equal, to wave height. Said obtained energy is equal to the force on the piston of the conveeter, which is substantially constant, times the piston's stroke, which is substantially directly proportional to wave height.
This is not in general the best amount of energy to obtain from a wave. The amount of power being delivered by a succession of waves, which one would like to obtain, is proportional, other things equal, to the square of wave height. Hagen points this out and prescribes that his wave-to-hydraulic unit deliver an amount of liquid which is more than directly proportional to the relative pitch between adjacent hulls, thus more than directly proportional, other things equal, to wave height. Hagen also discloses and claims two different means for converting relative pitching of hinged hulls into hydraulic power which work as prescribed.
Thus Hagen succeeded in prescribing characteristics for a wave-to-hydraulic power conversion means which can not only be efficient itself, but also facilitates maintenance of peak efficiency throughout the several other, serial stages of power conversion performed by the system of which it is a part. This is an important step towards the practical production of power from waves.
It detracts little from this achievment to point out that the two particular means disclosed by Hagen appear to be less serviceable than the best prior art. The one shown in FIGS. 3-6, inclusive, uses large areas of flexible material in a flexing mode, while the other, FIGS. 7 and 8, puts heavy lateral loads on a hydraulic cylinder and piston.